DE-C-37 33 319 describes an apparatus for the scrubbing of flue gas which has a prescrubbing stage for the removal of hydrohalogen compounds and a gas scrubber following this prescrubbing stage which has three scrubbing zones. The intermediate or middle scrubbing zone is provided with a scrubbing liquid circulation utilizing as the scrubbing liquid or absorbent, ammonia water, i.e. a solution of NH.sub.3 in water. From the scrubbing liquid circulation, a partial stream is branched through an oxidation unit and in the oxidation unit ammonium sulfite from the scrubbing liquid and ammonium hydrogen sulfide contained therein are transformed into ammonium sulfate.
In the prescrubber water is introduced with recirculation of the scrubbing water from the prescrubber sump to a spray unit in an upper portion of the prescrubber. From the prescrubbing sump a partial stream is branched to the scrubbing liquid circulation of the intermediate zone of the scrubber following the prescrubber.
Toxic and/or noxious components which are separated out in the prescrubber are, therefore, transferred to the absorbent circulation of the scrubber unit downstream of the prescrubber and are discharged with the ammonium sulfate solution from the oxidation unit. The ammonium sulfate solution can be subjected to evaporation to concentrate the ammonium sulfate therein, and to granulate the ammonium sulfate which precipitates out as an unpurified end product. The recovered ammonium sulfate can be used as a fertilizer or in fertilizers.
EP-A 620 187 describes an apparatus that also has a prescrubber for removing the hydrohalogen substances and a scrubber following the prescrubber for removal of the SO.sub.2.
The scrubber sump is provided with sump aeration and the scrubbing liquid is primarily ammonium sulfate. The latter can be discharged from the scrubber and the scrubbing liquid within the scrubber can be discharged through spray nozzles. An ammonium sulfate partial stream is fed to the liquid sump of the prescrubber which forms a recirculation from the sump of the scrubber. Recirculation permits a concentration or thickening of the ammonium sulfate solution. Part of the thickening ammonium sulfate solution is withdrawn from the prescrubber sump, thickened by hydrocyclones and dried to from the ammonium sulfate which can be used as or in a fertilizer. The end product here is also contaminated by any noxious or toxic substances which can separate out in the prescrubber.
Apparatus having separate prescrubbers for the removal of hydrohalogen compounds and special gas scrubbers for the removal of SO.sub.2 from such combustion gases are expensive. It is also a disadvantage with conventional apparatus that only a contaminated or impure end product can be obtained and even that end product may not be satisfactory as a fertilizer.